Real-Time Monitoring of Formation and Dynamics of Intra- and Interchain Phases in Single Molecules of Polyfluorene

Poly(9,9-dioctylfluorene) (PFO) is one of the most important conjugated polymer materials, exhibiting outstanding photophysical and electrical properties. PFO is also known for a diversity of morphological phases determined by conformational states of the main chain. Our goal in this work is to address some of the key questions on formation and dynamics of one such conformation, the beta-phase, by following in real time the evolution of fluorescence spectra of single PFO chains. The PFO is dispersed in a thin polystyrene film, and the spectra are monitored during the process of solvent vapor annealing with toluene. We confirm unambiguously that the PFO beta-phase segments are formed on a true single-chain level at room temperature in the solvent-softened polystyrene. We further find that the formation of the beta-phase is a dynamic and reversible process occurring on the order of seconds, leading to repeated spontaneous transitions between the glassy and beta-phase segments during the annealing. Comparison of PFO with two largely different molecular weights (M-w) shows that chains with lower Mw form the beta-phase segments much faster. For the high M-w PFO chains, a detailed Franck-Condon analysis of the beta-phase spectra shows a large distribution of the Huang-Rhys factor, S, and even dynamic changes of this factor occurring on a single chain. Such dynamics are likely a manifestation of changing coherence length of the exciton. Further, for the high Mw PFO chains we observe an additional conformational state, a crystalline gamma-phase. The gamma-phase formation is also a spontaneous reversible process in the solvent-softened matrix. The phase can form from both the beta-phase and the glassy phase, and the formation requires high M-w to enable intersegment interactions in a self-folded chain.